printk: Implement some unlocked kmsg_dump functions

If used from KDB, the locked variants are prone to deadlocks (suppose we
got to the debugger w/ the logbuf lock held).

So, we have to implement a few routines that grab no logbuf lock.

Yet we don't need these functions in modules, so we don't export them.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 71 insertions, 13 deletions

diff --git a/include/linux/kmsg_dump.h b/include/linux/kmsg_dump.h
index d6bd50110ec2..2e7a1e032c71 100644
--- a/include/linux/kmsg_dump.h
+++ b/include/linux/kmsg_dump.h
@@ -55,12 +55,17 @@ struct kmsg_dumper {
 #ifdef CONFIG_PRINTK
 void kmsg_dump(enum kmsg_dump_reason reason);
 
+bool kmsg_dump_get_line_nolock(struct kmsg_dumper *dumper, bool syslog,
+                               char *line, size_t size, size_t *len);
+
 bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
                         char *line, size_t size, size_t *len);
 
 bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
                           char *buf, size_t size, size_t *len);
 
+void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper);
+
 void kmsg_dump_rewind(struct kmsg_dumper *dumper);
 
 int kmsg_dump_register(struct kmsg_dumper *dumper);
@@ -71,6 +76,13 @@ static inline void kmsg_dump(enum kmsg_dump_reason reason)
 {
 }
 
+static inline bool kmsg_dump_get_line_nolock(struct kmsg_dumper *dumper,
+                                             bool syslog, const char *line,
+                                             size_t size, size_t *len)
+{
+        return false;
+}
+
 static inline bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
                                 const char *line, size_t size, size_t *len)
 {
@@ -83,6 +95,10 @@ static inline bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
         return false;
 }
 
+static inline void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper)
+{
+}
+
 static inline void kmsg_dump_rewind(struct kmsg_dumper *dumper)
 {
 }
diff --git a/kernel/printk.c b/kernel/printk.c
index c8129678dfbf..ac4bc9e79465 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -2510,7 +2510,7 @@ void kmsg_dump(enum kmsg_dump_reason reason)
 }
 
 /**
- * kmsg_dump_get_line - retrieve one kmsg log line
+ * kmsg_dump_get_line_nolock - retrieve one kmsg log line (unlocked version)
  * @dumper: registered kmsg dumper
  * @syslog: include the "<4>" prefixes
  * @line: buffer to copy the line to
@@ -2525,11 +2525,12 @@ void kmsg_dump(enum kmsg_dump_reason reason)
  *
  * A return value of FALSE indicates that there are no more records to
  * read.
+ *
+ * The function is similar to kmsg_dump_get_line(), but grabs no locks.
  */
-bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
-                        char *line, size_t size, size_t *len)
+bool kmsg_dump_get_line_nolock(struct kmsg_dumper *dumper, bool syslog,
+                               char *line, size_t size, size_t *len)
 {
-        unsigned long flags;
         struct log *msg;
         size_t l = 0;
         bool ret = false;
@@ -2537,7 +2538,6 @@ bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
         if (!dumper->active)
                 goto out;
 
-        raw_spin_lock_irqsave(&logbuf_lock, flags);
         if (dumper->cur_seq < log_first_seq) {
                 /* messages are gone, move to first available one */
                 dumper->cur_seq = log_first_seq;
@@ -2545,10 +2545,8 @@ bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
         }
 
         /* last entry */
-        if (dumper->cur_seq >= log_next_seq) {
-                raw_spin_unlock_irqrestore(&logbuf_lock, flags);
+        if (dumper->cur_seq >= log_next_seq)
                 goto out;
-        }
 
         msg = log_from_idx(dumper->cur_idx);
         l = msg_print_text(msg, 0, syslog, line, size);
@@ -2556,12 +2554,41 @@ bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
         dumper->cur_idx = log_next(dumper->cur_idx);
         dumper->cur_seq++;
         ret = true;
-        raw_spin_unlock_irqrestore(&logbuf_lock, flags);
 out:
         if (len)
                 *len = l;
         return ret;
 }
+
+/**
+ * kmsg_dump_get_line - retrieve one kmsg log line
+ * @dumper: registered kmsg dumper
+ * @syslog: include the "<4>" prefixes
+ * @line: buffer to copy the line to
+ * @size: maximum size of the buffer
+ * @len: length of line placed into buffer
+ *
+ * Start at the beginning of the kmsg buffer, with the oldest kmsg
+ * record, and copy one record into the provided buffer.
+ *
+ * Consecutive calls will return the next available record moving
+ * towards the end of the buffer with the youngest messages.
+ *
+ * A return value of FALSE indicates that there are no more records to
+ * read.
+ */
+bool kmsg_dump_get_line(struct kmsg_dumper *dumper, bool syslog,
+                        char *line, size_t size, size_t *len)
+{
+        unsigned long flags;
+        bool ret;
+
+        raw_spin_lock_irqsave(&logbuf_lock, flags);
+        ret = kmsg_dump_get_line_nolock(dumper, syslog, line, size, len);
+        raw_spin_unlock_irqrestore(&logbuf_lock, flags);
+
+        return ret;
+}
 EXPORT_SYMBOL_GPL(kmsg_dump_get_line);
 
 /**
@@ -2664,6 +2691,24 @@ out:
 EXPORT_SYMBOL_GPL(kmsg_dump_get_buffer);
 
 /**
+ * kmsg_dump_rewind_nolock - reset the interator (unlocked version)
+ * @dumper: registered kmsg dumper
+ *
+ * Reset the dumper's iterator so that kmsg_dump_get_line() and
+ * kmsg_dump_get_buffer() can be called again and used multiple
+ * times within the same dumper.dump() callback.
+ *
+ * The function is similar to kmsg_dump_rewind(), but grabs no locks.
+ */
+void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper)
+{
+        dumper->cur_seq = clear_seq;
+        dumper->cur_idx = clear_idx;
+        dumper->next_seq = log_next_seq;
+        dumper->next_idx = log_next_idx;
+}
+
+/**
  * kmsg_dump_rewind - reset the interator
  * @dumper: registered kmsg dumper
  *
@@ -2676,10 +2721,7 @@ void kmsg_dump_rewind(struct kmsg_dumper *dumper)
         unsigned long flags;
 
         raw_spin_lock_irqsave(&logbuf_lock, flags);
-        dumper->cur_seq = clear_seq;
-        dumper->cur_idx = clear_idx;
-        dumper->next_seq = log_next_seq;
-        dumper->next_idx = log_next_idx;
+        kmsg_dump_rewind_nolock(dumper);
         raw_spin_unlock_irqrestore(&logbuf_lock, flags);
 }
 EXPORT_SYMBOL_GPL(kmsg_dump_rewind);